Attenuation of optical signal in optical fibers caused by the fiber or cable bending has been one of the major concerns in fiber, cable and photonic device manufacturing. There are an increasing number of applications where optical fibers need to be routed through constrained spaces or where fibers need to be matched or mounted tightly to structures having arbitrary shapes and forms. In such environments, a significant number of tight bends can be expected down the transmission fiber route. Fibers with high bend tolerance are therefore required in such environments. Typical examples are in-house and in-building wiring in fiber-to-the-home (FTTH) systems, fly-by-light wiring systems in modern aircrafts, sensor applications where optical sensors connect to signal processors through optical fibers, and other similar applications. There are also various optical devices and systems like optical amplifiers, fiber lasers, optical delay lines and pay-out systems for remotely guided autonomous vehicles or projectiles that use substantial lengths of coiled optical fibers. Sizes of such devices are limited by the bend loss tolerance of optical fibers used. Better bend loss tolerant fiber allows for construction of smaller and more compact devices.
A large portion of the work relating to improvement of bend loss sensitivity has been carried out for single-mode fibers and fiber systems. The improvement of bend tolerance in single-mode fiber may be achieved through the fiber refractive index profile of double cladding structures, as described for example in U.S. Pat. No. 4,838,643. Similar approaches based on the shaping of the outer part of the core are described in U.S. Pat. No. 5,278,931 and U.S. Pat. No. 6,771,865. Index depression (index trench) may also be used to improve fiber bend tolerance. Variations of these solutions are presented in U.S. Pat. No. 4,852,968, U.S. Pat. No. 5,032,001, U.S. Pat. No. 6,901,196, and U.S. Pat. No. 6,947,652. Many of the presented solutions reduce the mode field diameter of the fundamental mode, when compared to standard single mode fiber such as Corning SMF-28, resulting in an increased loss when bend resistant fibers are spliced to standard telecommunication fiber. Alternative solutions are proposed in U.S. Pat. No. 5,175,785 and U.S. Pat. No. 6,711,330 where few-mode or even multimode fibers are used to transmit the light with fundamental mode of the fiber and thereby provide single mode operation of the transmission link. In the latter case, the mode field can be relatively large, while providing good bend tolerance of the fiber transmission system. While the transmission channel is single mode, the presence of higher order modes cannot be completely excluded, which prohibits application of such systems in various single-mode applications. Another solution, that allows single-mode and nearly single-mode operation of the fiber while maintaining large mode field diameter of the fundamental mode, relies on application of photonic crystal fibers, more precisely on the introduction of air holes around the core of the fiber. This concept is presented in U.S. Pat. No. 6,856,742, WO01/37008, WO2007034923 and U.S. Pat. No. 7,142,757.
As briefly summarized in the previous paragraph, there are various methods available in the art that can provide high bend tolerance of single mode fibers. However, there are only few and limited solutions that can improve bend tolerance of multimode transmission systems. Multimode fibers are widely deployed in short range optical transmission applications. Local area networks, short range industrial wiring, short range computer to computer communications, optical sensor interconnections and military applications often rely on multimode fibers as they provide easier and more reliable connectivity and lower cost terminal equipment. In such applications, fibers are in many cases routed through numerous constricted space areas and this implies the presence of tight bends.
There are several serious limitations that restrict design opportunities for bend resistant multimode fibers. Since multimode fibers support a large number of modes and each mode bears individual bend loss characteristics, it is difficult to control bend loss and other waveguide properties of all propagating modes at once. In general, groups of modes that have high effective refractive index also exhibit high bend tolerance, while groups of modes with low effective refractive index show poor bend performance. Furthermore, it is often desired that the multimode fiber profile minimizes differences in group velocities between propagating modes, otherwise the modal dispersion and thereby bandwidth of the transmission multimode fiber can become severely degraded. This is especially true of high bandwidth transmission multimode fibers where even minor intervention into the optimum shape of the graded index profile inevitably leads to serious degradation of transmission fiber bandwidth. Finally, it is also desired that bend resistant multimode fiber exhibits good compatibility with existing standard multimode fibers and terminal equipment to allow for effective and economical interconnectivity.
A couple of known approaches for bend-tolerant, multimode fiber designs are presented for example in patent applications JP 20060785543 and JP 2006047719. Both of these approaches employ a trench concept that helps confine the modes within the core and increases mode field roll-off in the cladding, which reduces higher order mode power leakage in the cladding in the bent fiber. The trench depth and width is limited by the production process in silica fibers and also by propagation of spurious modes guided by the trench that can severely degrade the fiber performance. Location of the trench close to the fiber core also affects higher order mode group velocities that can result in significant degradation of fiber bandwidth. Approaches presented in other known art include holey assisted multimode fiber design where series of holes is placed around the core to improved guidance of the light within the core as presented in US 20060034574. The holey fibers are difficult to splice, and the high index contrast holes can compromise the fiber bandwidth and can induce additional guidance losses while providing limited improvement in bend loss performance.